This invention relates to a method and apparatus for coating and more particularly, to a method and apparatus pertaining to "fountainless" coating. As such, it constitutes an improvement on my prior U.S. Pat. No. 3,695,221.
In that patent, apparatus was described which reduced foaming in a fountainless, reverse angle doctor blade coater by promoting fast flow of the coating liquid to minimize the shearing action developed by the reverse angle doctor blade. This was achieved by employing a baffle or back wall of the liquid holding chamber closely adjacent the coating roll. The back wall was equipped with end walls that directed outflow of the excess liquid along the end faces of the coating roll. Wipers were installed to remove the coating liquid from the end faces to prevent liquid buildup and possible fouling of the journals. Although the apparatus of the patent was successful in eliminating the problem of foaming of the coating liquid (which could result in splotchy coating or printing), the artisans operating the machinery failed to maintain the wipers in proper fashion. For example, the wipers were not replaced when worn or not adjusted properly with the result that the coating liquid flowed along the roll end faces onto the journals and ultimately into the bearings.
To remedy this, I installed barrier strips between the chamber end walls and the roll end faces to prevent liquid flow onto the roll end faces. This resulted in the reduction of flow so that in some cases that the liquid tended to entrap air reaching a milk-shake consistency, viz., foaming.
To overcome the disadvantage of this expedient, I bored a hole in the lower portion of each end wall to achieve a higher flow rate of excess coating liquid and reduce the undesirable foaming. However, with the higher inputs to achieve higher flow rates, the liquid levels reached heights sufficient to spill over the barrier strips and again foul the journals.
I then decided that the end walls had to be eliminated to permit rapid outflow of liquid and thus avoid fouling of the roll end faces and journals. This was less than optimum because the coating was not uniform along the length of the coating roll. More particularly, it appeared that the fast outflow of liquid from the liquid holding chamber (an open-ended chamber) resulted in a liquid level that was not uniform across the face of the roll. Thus, the end portions of the roll and hence the edges of the web being coated were literally "starved" of the coating liquid.
I then experimented with a partial end wall, i.e., one that did not completely close the gap but this, although correcting the liquid level problem, brought about again the problem of roll end face fouling. This situation was further aggravated by the fact that liquid would build up in the form of a bead or annulus at the end of the roll and thus leave an unacceptable trace on the web.
I then discovered that the problems of foaming, nonuniformity of coating, undesirable beading and roll end face fouling all were solved by interposing a restriction to the horizontal outflow of excess coating liquid at a critical location within the fountainless chamber. In particular, the restriction is located slightly inwardly of the ends of the roll and provides a substantially uniform liquid level in contact with the roll along the length thereof while eliminating foaming, beading and roll end face fouling. The latter is particularly inportant because the wipers could be omitted.
Other details of the invention including other objects and advantages may be seen in the ensuing specification.